The present invention relates to the drying of solvents-containing materials in a vacuum concentrator and, more particularly, to nearly complete recovery of the solvent in a cold trap by means of cryopumping effect existing between the concentrator and cold trap with the result that only insignificant amounts of solvent can pass from the system into the system vacuum pump.
Our pending application Ser. No. 07/549,447, filed Jul. 6, 1990, now U.S. Pat. No. 5,137,604, granted Aug. 11, 1992, discloses a vacuum drying system in which solvent vapors evolve from solvents-containing material in a concentrator or vacuum chamber evacuated by a vacuum pump. These solvent vapors feed from the concentrator into a condensation cold trap located in a conduit connecting the concentrator and pump, the trap being intermediate the locations of the concentrator and pump. A valve is located between the outlet of the cold trap and the inlet to the vacuum pump, and opening and closing of this valve is microprocessor controlled in accordance with vacuum condition in the concentrator.
When the vacuum condition, i.e., absolute pressure in the concentrator is at a certain value, the microprocessor closes the valve and pump effect on the system is interdicted so that solvent cannot be drawn beyond the cold trap and into the vacuum pump. By keeping solvent from entering the vacuum pump, adverse effect on pump lubricant and/or internal pump structure is minimized. Interdiction function of the valve will be present during most of the drying cycle, but solvent will continue to pass from the concentrator to the cold trap by reason of cryopumping effect existing because of difference between the vapor pressure of the solvent in the concentrator and a lower absolute pressure in the cold trap.
When an absolute pressure rise occurs in the system that would weaken system cryopumping conditions, the control unit will signal opening of the valve to connect the pump with the chamber and until a certain condition of vacuum or lowered absolute pressure is restored in the chamber at which time, the control unit will signal closing of the valve so that continued evolution of the solvent from a material being dried in the chamber can proceed solely by way of cryopumping.
While this system works quite effectively for the intended purpose of protection of the pump and its lubricant, and for complete and reasonably timed solvents-containing material drying, it requires use of an expensive microprocessor control unit and devices that are all power operated.
Accordingly, it is desirable that drying apparatus of the type disclosed in our copending application be improved to achieve the intended apparatus purpose while using more simplified and inexpensive components in regard to the opening and closing of the valve which isolates or connects the cold trap with the vacuum pump.